I will be taking two cruises one that ends in Fort Lauderdale and the other starting the same day from Miami what is the best way to get between both ports

rent a car from Dollar, you can pick-up at FLL and drop-off at MIA, no drop-off charges for $30 plus some cents, taxes and fees included. Dollar will also pick you up/drop you off at each port at the start/end of your rental.

I will be taking two cruises one that ends in Fort Lauderdale and the other starting the same day from Miami what is the best way to get between both ports

Hello Sandra,

You have a few choices:
Taxi Cab (around $85 to $90).
Van 12 passengers type shuttle ($20 to $22) you may need a least 5 people to get that price.
Private transportation: some time they used vans or SUVs. In general is a great services they charge a flat fee around $90 to $120. The prices depends more in the quality of the services, serious companies will have a car waiting for you and usually the ask for a credit card number to take your reservation if you change your mined in last minute and take other type of transportation they will charge you. The ones the charge a cheaper fare what they do they go at the pier to wait for you what if the get a better ride for more money they will forgot about you.
Limousine services also are available that will be around $200 to $300.